1. Field of the Invention
The present invention relates to an image input/output apparatus for inputting/outputting image information.
2. Description of the Prior Art
A focusing optical system having an optical element of a large aperture size is required to optically read an image having a high resolution, a high magnification, and a high luminance. Conventional focusing optical systems represented by lenses have smaller focal depths when their aperture sizes are increased. In the fields of applications which utilize image equipment such as microscopes, cameras, and endoscopes, the resultant images preferably have high resolutions and luminances and at the same time have large focal depths.
A conventional technique for obtaining an image having a large focal depth is described in "W. T. Welford, Journal of Optical Society of America, Vol. 50, P. 749 (1960)". This technique describes a means which utilizes a focusing optical system having a special aperture such as a zone plate. According to this means, an image having a large focal depth can be advantageously obtained. However, the resolution and the light amount are greatly decreased, resulting in inconvenience.
Another technical means is a method which uses an image processing technique described in "S. A. Sugimoto and Y. Ichioka, Applied Optics Vol. 24, P. 2076 (1985)" or "Takatoshi OTA, Kokichi SUGIHARA, and Noboru SUGIE, Journal of the Institute of Electronic and Communication Engineers (D), J66-D, 1245, (1983)". According this method, a plurality of images having different focal points (to be referred to as in-focus positions) in an object space to obtain local dispersion information, and portions in the in-focus state are synthesized. According to this means, an image having a large focal depth can be synthesized without decreasing the resolution and luminance. The above means can be deemed as an effective means in this respect. However, when an object has a smooth surface or different projections at different positions, it is difficult to apply the above method to such an object. In addition, large hardware is required to realize the method, and an algorithm is complicated, thus posing many practical problems.
The conventional technical means described above have the following drawbacks.
(1) In the means for increasing the focal depth by utilizing a focal optical system having a special aperture such as a zone plate, the resolution and the light amount are greatly decreased. Although it is possible to obtain an image having a large focal depth, the resolution and luminance of the image are lost. Therefore, this mean cannot serve as an optical system which satisfies all factors, i.e., a focal depth, a resolution, and a luminance.
(2) In the means for inputting a plurality of images having different in focus positions and synthesizing partial images of the in-focus state on the basis of the local dispersion information, applications are limited depending on conditions of the objects, thus posing practical problems.